The present invention relates to a system for controlling idle speed of an automotive engine having an electronic fuel-injection system, and more particularly to a control system for the fast idle at cold-engine operation.
In an idle speed control system for an electronic fuel injection system, a bypass having an idle speed control valve is provided around a throttle valve of the engine. The idle speed control valve is operated to control the amount of intake air for maintaining a desired idle speed by a feedback control system. The desired idle speed is derived from a memory based on input signals representing engine operating conditions. Namely, the opening degree of the idle speed control valve is controlled in accordance with various engine driving conditions such as starting of the engine, fast idle at cold-engine operation, and transient state.
When the engine is cold, the response speed of the engine is slow. Accordingly, if the opening degree of the idle speed control valve is increased to increase the amount of fuel so as to control the idle speed to the desired idle speed, the engine speed does not increase immediately. Thus, if the feedback control continues, hunting of the system may occur.
Hence, in general, under such driving conditions, the engine speed is controlled by the open-loop control to restrict the hunting. However, if the engine speed reduces at cold-engine operation, the reduced engine speed is not recovered, which causes the engine to stall.
On the other hand, Japanese Patent Application Laid-Open 59-3135 discloses a feedback control system for controlling the idle speed control valve at cold-engine operation. In the system, integral term in a feedback value is reduced so that the engine speed slowly approaches the desired engine speed, thereby preventing the stall of the system. However, even if the effect of the feedback control is delayed, the hunting can not be prevented.